Lithography is widely used in various industrial applications, including the manufacture of integrated circuits, flat panel displays, micro-mechanical systems, micro-optical systems etc. Generally speaking, the lithography process is used for producing a patterned structure. During the manufacture of integrated circuits, a semiconductor wafer undergoes a sequence of lithography-etching steps to produce a plurality of spaced-apart stacks, each formed by a plurality of different layers having different optical properties. Each lithography procedure applied to the wafer results in the pattern on the uppermost layer formed by a plurality of spaced-apart photoresist regions.
To assure the performance of the manufactured products, the applications of the kind specified above require an accurate control of both the dimensions of sub-micron features of the obtained pattern and the roughness of the lines. The most important parameter for lithography, usually termed “critical dimension” (CD), is the smallest transverse dimension of the developed photoresist, usually being the width of the finest lines or the width of the smallest spaces between lines. Since the topography of the measured features is rarely ideal additional information found in the height profile, such as slopes, curves etc., may also be valuable in order to improve the control of the fabrication process. However, the shape of the lines in never identical when looking at different locations along the lines nor between different lines. For a full description it is therefore required to treat the line profile (even it for simplicity, we focus only on one profile parameter, e.g. the CD at the bottom of the line), as an ensemble of values rather than a single value. There are accordingly two kinds of measurements that can be done: a local measurement, relevant only to a specific line at a specific point along the line (such as usually done by e.g. CD-AFM or CDSEM), and an average measurement that averages some area along the lines and including several such lines (such as done by optical methods). When using a measurement that samples an area, such as optical methods, it is usually assumed that the measured value is some average of the specific local values. In many cases measuring the average value is sufficient or even advantageous over measuring local values, as it is assumed that most macroscopic process parameters affect the average profile while the noise contained in the local measurement due to line edge roughness (LER) is better averaged out However, the LER can be ignored as “noise” only as long as it is much smaller than the average profile parameters. If LER becomes a significant fraction of the CD, as may happen, for example, if average CD is reduced while LER is kept at its original magnitude (see FIG. 1), or due to some microscopic property of lithography process that promotes a large LER, than LER become an important characteristic of the process that has to be monitored. As shown in FIG. 1, the same line edge roughness (LER) is added to two different lines with different average CDs, CD1 and CD2 accordingly. The ratio of the thinnest to the widest point is obviously making the LER a major issue for the thinner line whereas it may be ignored for the wider line.
Since LER is basically a complementary (“error bar”) measurement to CD measurement, it would be advantageous to have the two being measured within the same tool. Such a measurement would be relatively straightforward for a SEM, which measures local values, however not as simple to an optical measurement tool (Scatterometry), that by nature measure average values over an area. In this invention we describe several ways by which an optical tool, can be utilized or enhanced in order to measure LER.